1. Field of the Invention
The present invention relates to a microwave plasma processing apparatus used in the production of semiconductor devices for dry etching or forming a thin film by CVD (Chemical Vapor Deposition) by utilizing microwave plasma and the operation method thereof.
2. Description of the Prior Art
The microwave plasma processing apparatuses for processing the surface of a substrate by utilizing ECR (Electron Cyclotron Resonance) plasma has attracted much attention in the field of semiconductor device manufacture. One of the most important features of ECR plasma processing resides in the fact that the electrons are accelerated by the resonance effect between the magnetic field and the microwaves so that the kinetic energy of the accelerated electrons ionizes a gas, thereby creating high-density plasma. Each of the electrons excited by the microwaves makes a rotary motion about a line of magnetic force. In this case, the condition that the centrifugal force and Lorenz force are balanced is defined as the ECR condition. This condition is expressed by EQU .omega./B=q/m
where the centrifugal force and the Lorenz force are expressed by mr.multidot..omega..sup.2 and -qr.multidot..omega.B, respectively, wherein,
.omega.: angular frequency of the microwaves; PA1 B: magnetic flux density; and PA1 q/m: specific charge of electron.
In general, the microwave frequency is 2.45 GHz which is industrially accepted. In this case, the resonance magnetic flux density is 875 gauss.
FIG. 1 illustrates a sectional view used to explain the construction of a conventional ECR plasma apparatus. The microwaves generated by a microwave generator (not shown) are introduced into a plasma generation chamber 3 through a waveguide 1. A gas such as N.sub.2, O.sub.2, Ar or the like for generating plasma is introduced through a gas supply pipe 4 into the plasma generation chamber 3. Disposed between the waveguide 1 and the plasma generation chamber 3 is a vacuum window 2 such as quartz in order to gas-tightly separate the waveguide 1 under atmospheric pressure and the plasma generation chamber 3 which is evacuated by an evacuation system (not shown). Also disposed at the lower end of the plasma generation chamber 3 is a metal plate 7 with a large opening 7A. This plate 7 and the plasma generation chamber 3 define a half-opened microwave resonator. Excitation solenoid 6 surrounds the outer surface of the resonator in such a way that a magnetic field adapted to satisfy the ECR condition is generated, whereby plasma is produced within the resonator chamber. The plasma thus generated is forced into a processing chamber 9 along the lines of magnetic force and is directed toward a substrate stand 10. For example, monosilane gas (SiH.sub.4) is introduced into the processing chamber 9 through a gas supply means including a valve 12A and a supply pipe 12 so that the introduced gas is activated by the plasma. Then, the activated species react with a substrate 11 which is a specimen to be processed, whereby a thin film is formed over the surface of the substrate. When, an etching gas is supplied through the gas supply pipe 4 instead of N.sub.2 or the like the apparatus can be used for etching the surface of a substrate.
Prior to the description of the problems resulting from the construction and the operation of the ECR plasma etching apparatus or the CVD apparatus of the type described above, how a plasma is generated will be described. In the case of the ECR plasma etching apparatus or the CVD apparatus, in order to carry out the efficient etching or growth of a thin film by increasing the plasma density, a magnetic field region which satisfies the ECR condition must be established within the plasma generation chamber. However, since the length of the axial direction of the excitation solenoid is limited this magnetic field region is impossible to expand into the whole space of the plasma generation chamber as in the case of the length of the solenoid in the axial direction being limitless. The above-mentioned magnetic field region exists within a limited space only and the shape and the position of the region in the axial direction of the plasma generation chamber are determined in response to the outer and inner diameters, the height, a number of turns of the solenoid and other design factors and the magnitude of the current flowing through the excitation solenoid. Furthermore, the generation of the plasma is dependent upon the outer product of the strength of electric field and the magnetic flux density (E.times.B), so that the position of the magnetic field region (termed as the resonance magnetic field region hereinafter in this specification) relative to the microwave electric field strength distribution becomes a very important factor influencing the speed of the substrate surface processing as well as the quality of the processed surface, for example, the growth rate of a thin film over the surface of a substrate and film qualities such as thickness distribution of the film grown and the density of film. The microwave electric field strength distribution within the plasma generation chamber is dependent upon the shape and size of the plasma generation chamber and the matching condition between the microwaves and the load. As an example of a microwave electric field strength distribution, FIG. 2 illustrates a schematic view of a microwave electric field strength distribution when the plasma generation chamber is constructed as a resonator creating the resonance mode TE.sub.113. In this case, within the plasma generation chamber 3, there exist three crests of the standing microwave. The electric field strength in the axial direction of the plasma generation chamber is equal to the amplitude of the standing wave and which is decreased in the radial direction of the chamber. It is thought, therefore, that when the space in which the resonance magnetic field region is created is controlled by varying the magnitude of the current flowing through the solenoid, the ECR plasma generation efficiency as well as the distribution of the plasma density within the plasma generation chamber can be controlled. So far based only on the above described technical idea, only the solenoid current is controlled to determine an optimum magnitude so that the overall characteristics of the quality of a thin film, the film growth rate, the distribution of the thickness of the grown film, become optimum, but there exists the problem that the optimum overall characteristics cannot be attained when only the solenoid current is controlled.